1. Technology Field
The present invention relates to optoelectronic devices. More particularly, the present invention relates to systems and methods for connecting optoelectronic subassemblies to electrical circuits, for example, a printed circuit board.
2. The Related Technology
Fiber-optic and optoelectronics are an important aspect of modern networking circuits because they allow for efficient, accurate, and rapid transmission of data between various components in the network system. As with the design of most systems, design considerations often determine the extent of use of a fiber optic system. For example, the size and modularity of components or devices must often be balanced against the need for additional space to accommodate heat dissipation and circuit monitoring components. While it is desirable to minimize a component's size, some design considerations have previously limited this minimization due to their inherent characteristics. For example, some optoelectronic components generate large amounts of heat, which becomes more difficult to dissipate as the size of the component becomes smaller. Further, as the component becomes smaller, there is less space available for mounting and connecting additional components thereto.
Modular components are desirable in fiber optic systems to reduce the cost of manufacturing the system, which cost increases the more customized the system becomes. An example of a modular component is an optical transceiver module (“transceiver”). Transceivers usually include an input receiver optical subassembly (“ROSA”) and an output transmitter optical subassembly (“TOSA”). The ROSA includes a photodiode for detecting optical signals and sensing circuitry for converting the optical signals to digital electrical signals compatible with other network components. The TOSA includes a light source, such as a laser, for transmitting optical signals and control circuitry for modulating the laser according to an input digital electrical data signal. The TOSA also includes an optical lens for collimating the light signals from the laser of the TOSA to an optical fiber. Additionally, the transceiver includes pluggable receptacles for optically connecting the TOSA and the ROSA with other components within a fiber optic network. The transceiver further includes an electronic connector for mating with a host system, such as a computer or communication device, with which the transceiver operates.
As mentioned, photodiodes and lasers are employed in the ROSA and TOSA, respectively, and as such are examples of optoelectronic components. Generally, these optoelectronic components are sensitive electrical devices, and therefore require environmental protection. In response to this need, the photodiode and laser are usually positioned in packaging assemblies within the respective ROSA or TOSA. One such packaging assembly is known as a transistor-outline header or transistor-outline package, referred to herein as a “TO package” or “TO can.” TO packages are widely used in the field of optoelectronics, and may be employed in a variety of applications. As such, the size of TO packages is often standardized to facilitate their incorporation into optoelectronic devices, such as ROSAs and TOSAs. TO packages protect the sensitive components contained therein and electrically connect such devices to external components such as printed circuit boards (“PCBs”), which are also located in the transceiver.
With respect to their construction, TO packages often include a cylindrical metallic base with a number of straight conductive leads extending therethrough in an arrangement that is generally perpendicular to the base. The size of the base and its respective lead configuration is typically designed to fit within one of a variety of standard form factors, such as TO-5 or TO-46 form factors, for instance. The TO package leads are usually hermetically sealed in the base in such a way as to provide mechanical and environmental protection for the components contained in the TO package, and to electrically isolate the leads from metallic portions of the base. Typically, one of the conductive leads is a ground lead that may be electrically connected directly to the base.
Various types of electrical devices are mounted on an interior surface of the TO package base and connected to the leads. Generally, a cover is used to enclose this interior surface where such electrical devices are mounted, thereby forming a chamber with the base that helps prevent contamination or device damage.
The particular design of the TO package depends on both the type of optoelectronic device that is mounted on the base and the configuration of the modular component with which the TO package will operate. By way of example, in applications where the optoelectronic device mounted on the base is an optical device such as a laser or photodiode, the cover of the TO package includes a transparent optical window so to allow an optical signal generated or received by the optical device to be transmitted to or from the TO package. These optical TO packages are also known as window cans.
Certain challenges exist when connecting the TO package to other components of a system or device, such as the transceiver discussed above, in which the TO package is located. For example, the conductive lead configuration of a conventional TO package limits how the package, and ultimately, the modular component to which it is associated, is connected to other components of the transceiver, e.g., the PCB. One attempt at solving this problem has involved positioning the TO package on its side such that the base thereof is perpendicular to the PCB surface. This configuration may be desirable where the optical window is disposed at an end of the can for the emission or reception of an optical signal. The leads of the TO package in this configuration linearly extend from the package base and straddle opposing surfaces of the PCB such that some of the leads are positioned adjacent solder pads located on a top surface of the PCB while other leads are positioned adjacent solder pads on a bottom PCB surface. The leads are soldered to the PCB pads in a surface-mount configuration. So configured, the TO package can be disposed in a TOSA or ROSA port to form part of a respective TOSA/ROSA subassembly of the transceiver.
Various challenges arise with the above TO package configuration as well, however. For instance the spacing of the leads exiting the TO package can sometimes vary such that it fails to match the thickness of the printed circuit board. Generally, the leads of the TO package ideally should rest just adjacent to and along the length of the solder pads of the PCB. Frequently, because of this mis-match between the TO package lead spacing and PCB thickness, however, the ends of the leads must be specially manipulated so that the lead ends can suitably lie parallel and against the corresponding PCB pads. This requires either very specialized tooling or manual lead forming. If done manually, lead forming frequently results in imperfect and irregular solder joints of poor quality. Further, such manual lead forming operations can crack or otherwise damage the glass-to-metal seals used to enable passage of the leads through the TO package base. Manual lead forming operations also result in additional assembly cost.
TO packages having leads as explained above can result in other challenges as well: the TO package leads in this configuration are unsupported between the base and the PCB and are thus unable to withstand torque, gravitational, jostling, or other displacement forces that may be imposed on the leads during use. Failure of the TO package leads, breakage of the solder bond between the leads and the PCB, and/or failure of the PCB pads are the likely result. In addition, stress applied to the TO package and its leads can translate through the relatively rigid PCB to damage other PCB-connected components.
Another approach in attempting to resolve some of the challenges discussed above has involved the connection of TO package leads to PCB pads using a flexible circuit. In this configuration, a TO package is positioned such that its base is perpendicular with respect to the top surface of the PCB. A flexible circuit electrically interconnects electrically contacts on the TO package base with pads on the surface of the PCB. One advantage of this configuration includes the ability of the flexible circuit to allow the optical subassembly in which the TO package is located to “float” slightly in the transceiver housing so that, when a connectorized optical fiber is plugged into a port associated with the optical subassembly, the port and attached optical subassembly can self-align so that it connects to the fiber properly.
Notwithstanding their utility, flexible circuits add significant cost to transceiver assembly, both in terms of extra materials and additional steps required for flexible circuit attachment to the PCB and TO package. In particular, flexible circuit connection to the contacts of the TO package must typically be performed via semi-manual soldering using a hot bar process. For connection with the PCB, solder paste is first stenciled atop pads on the PCB surface before corresponding pads on the flexible circuit are mated with the PCB pads. Heat and pressure are then applied to the region to reflow the solder joint and solidify the pad connection. The interconnection so formed is referred to as a “blind solder joint.” As such, the connection must be x-rayed to determine whether the solder joint was properly formed. While portions of the above process can be automated, it nonetheless represents a significant portion of transceiver assembly time.
As seen by the above discussion, various known TO package-to-PCB interconnection schemes suffer from various challenges, including propensity for stress-induced damage, and labor-intensive assembly procedures. Thus, a need exists for a connection scheme that enables electronic device component interconnection without creating the above-described challenges. Any solution to this need should facilitate the connection of an optoelectronic component, such as a TO package, to a corresponding component, such as a printed circuit board, within an optical transceiver module. Such a solution should present a reliable and efficient interconnection scheme, thereby maximizing utility of the transceiver in a variety of operating conditions.